Pre-press color-proofing is a procedure that is used by the printing industry to create representative images of printed material without the high cost and time that is required to actually produce printing plates and set up a high-speed, high volume, printing press to produce an example of the intended image. These examples may require several corrections and be reproduced several times to satisfy customer requirements. The pre-press color-proofing process saves time and money getting to an acceptable finished product prior to producing printing plates.
Once an intended image is approved by the customer, films required for exposing printing plates are generated. These films are produced on a separate apparatus such as an imagesetter and the imagesetter recording film is used to prepare printing plates which are used to print finished copies in high volume.
An example of a commercially available image processing apparatus is shown in commonly assigned U.S. Pat. No. 5,268,708. This image processing apparatus forms an intended image on a sheet of thermal print media in which dye from a sheet of dye donor material is transferred to the thermal print media by applying thermal energy to the dye donor material.
The printhead on the image processing apparatus includes a plurality of lasers diodes which are tied to the printhead and are individually modulated to supply energy to the thermal print media corresponding to an information signal. A plurality of optical fibers are individually coupled to the laser diodes at one end and terminate as a fiber optic array at the other end. The printhead moves relative to the longitudinal axis of the vacuum imaging drum. The dye is transferred to the thermal print media as the radiation is transferred from the laser diodes by the optical fibers to the printhead and thus to the dye donor material. The radiation is converted to thermal energy in the dye donor sheet material.
The level of laser power determines the amount of dye transferred. To assure consistent proof-to-proof dye transfer as well as machine-to-machine consistency, it is important that a given input signal results in a consistent amount of dye transfer. To set this dye transfer to a desired level, the image processing apparatus incorporates sensor circuitry and a calibration feedback control loop for modulating laser output power. To provide a measured signal, the laser is positioned so that it directs a beam of light at a calibration sensor. This calibration sensor measures the power level that it detects and, in turn, provides a corresponding output signal to laser driver control circuitry. Based on the signal level received from the calibration sensor, the laser driver control circuitry adjusts the input signal that drives each laser to modulate the laser output power. The operator of the image processing apparatus can then verify that the desired output levels are produced by measuring density patches from an image produced on the same image processing apparatus.
Although current processing apparatus operation is satisfactory, there are some limitations. For example, the throughput, commonly expressed in number of intended images produced per hour is limited in part by the laser power level. Existing devices, for example, use imaging lasers with 200-250 mW output power. Increasing this power level to 400 mW or higher would allow the lasers to effectively deliver the same output energy in less time. This, in turn, would allow faster drum rotation and faster writing speeds, thereby increasing throughput.
A second limitation with the currently available processing apparatus is the reliability and power range of existing calibration sensor components. In order to measure high-energy laser power using economical components, the calibration sensor requires a reliable filter that attenuates laser radiation to much lower levels. The cost of sensors for measurement at full laser power would be prohibitive for commercial image processing devices. To attenuate the laser signal, existing devices employ relatively high-cost, sensitive components such as coated filters, for example, Inconel 2.5 Neutral Density (ND) lenses. These components have proved to be scratch-sensitive and are limited in their ability to attenuate higher levels of laser power. For example, if multiple diodes are simultaneously turned on at 200-250 mW, the resulting output power can burn through the protective coating, destroying the filter itself as well as the sensor it is designed to protect.
Another limitation with existing methods for laser calibration is that a relatively expensive sensor component, typically a photodiode, must be selected to handle a high input-power signal. Moreover, the sensor chosen must be matched closely to the level of attenuation that can be achieved, constraining sensor availability. Low-cost photodiode sensors are available, but these sensors measure signals at a lower power range than is currently achievable using existing equipment.
Yet another limitation with existing methods for writing laser measurement is the accuracy required for alignment and focus of the imaging laser relative to the sensor component. Each laser must be precisely positioned relative to the attenuating filter and sensor to assure accurate measurement. In an image processing apparatus employing multiple lasers, repeated, precise repositioning of the lens assembly are required for each individual laser during laser power measurement.
Existing methods for laser power measurement include use of an opto-acoustical converter, discussed in U.S. Pat. No. 4,344,172, and methods for a laser output control feedback loop are described in U.S. Pat. No. 4,899,348. Examples of ceramic materials used as wave guides in optical components are shown in U.S. Pat. No. 5,577,137 and optical diffusers are discussed in "Machinable Glass Ceramic: A Useful Optical Material," Applied Optics, Vol. 25, No. 11/1, June, 1986, p. 1726. Prior art shows ceramic material used for control of laser modulation by varying the ionization state of a ceramic element. See U.S. Pat. No. 4,889,414.
Thus, is seen that there are a number of areas for improvement in calibration systems for lasers used in image processing apparatus. In particular, there is a need for low cost filters capable of withstanding high powered lasers and durable enough for repeated use, and which are relatively insensitive to precise positioning of the laser and sensor.